Transmission electron microscopy (TEM) provides ultrahigh resolution imaging of samples, surpassing the diffraction limit offered by optical microscopy. Due to the ultrahigh vacuum environment associated with TEM, the imaging of liquid samples or samples (e.g., nanoparticles or biological samples) suspended in a liquid (i.e., in situ liquid TEM) is not straightforward. In recent years, the development of in situ flow cells generally comprising a thin spacer sandwiched between two chips with thin (less than about 100 nanometer (nm)) silicon nitride membranes has led the field of in situ liquid TEM. The liquid sample is isolated from the ultrahigh vacuum environment by the spacer and is imaged through the silicon nitride membrane. Metallic contacts can be used to produce electrical biases across the liquid sample.
In situ liquid TEM imaging has found applications in a wide number of fields and has been used to study chemical and electrochemical processes, biological structures, and nanoparticle growth and dynamics, all at the nanoscale. However, due to the thickness and material properties of silicon nitride, some of the in situ flow cells suffer from decreased resolution and contrast, charging effects, and increased damage to the sample from electrons scattered by the silicon nitride. The low thermal conductivity of silicon nitride can also result in heating of the sample.